Disposable absorbent products currently find widespread use in many applications. For example, in the infant and childcare areas, diapers and training pants have generally replaced reusable cloth absorbent articles. Other typical disposable absorbent products include feminine care products such as sanitary napkins, panty shields, or tampons; adult incontinence products; and health care products such as surgical drapes or wound dressings. A typical disposable absorbent product generally comprises a composite structure including a top sheet, a back sheet, and an absorbent core structure between the top sheet and back sheet. These products usually include some type of fastening system for fitting the product onto the wearer.
The use of water-swellable, generally water-insoluble absorbent materials, commonly known as superabsorbent polymers (“SAP”), in disposable absorbent personal care products is known. Such absorbent materials are generally employed in absorbent products in order to increase the absorbent capacity of such products while reducing their overall bulk. Such absorbent materials are generally present in absorbent products in the form of small particles and may be included in a fibrous matrix, such as a matrix of wood pulp fluff. A matrix of wood pulp fluff generally has an absorbent capacity of about 6 grams of liquid per gram of fluff. The superabsorbent materials generally have an absorbent capacity of at least about 10, preferably of about 20, and often of up to 100 times, and even up to 300 times, their weight in water. Clearly, incorporation of such superabsorbent materials in disposable absorbent products can reduce the overall bulk while increasing the absorbent rate and capacity of such products.
The absorbent products mentioned above, such as baby diapers, adult incontinence devices, and feminine hygiene products, may be made with a cellulose fiber fluff-based absorbent core sandwiched between a liquid pervious top sheet, which allows the unobstructed passage of fluid to the absorbent core, and a liquid impervious backing sheet usually of plastic material, which contains the absorbed fluid and prevents it from passing through the absorbent core and soiling the undergarments or clothing of the wearer of the absorbent article. The core product of the present invention does not require, but may contain, fibrous material. In various embodiments of the present invention, SAP powder is held within a container, with or without fibrous material. The container may optionally include openings on either of its surfaces that are pre-formed and/or formed during manufacture so as to admit liquids to become absorbed by the SAP powder or even to become collected in liquid form. As will be disclosed in more detail later, the openings created during manufacture of the absorbent product by the SAP particles during the pressing step are sufficient for most applications.
In recent years, market demand for thinner and more comfortable absorbent articles has increased. Ultra-thin feminine napkins are no longer constructed from loose wood pulp, which tends to give a bulky product, but with a roll wood-based air-laid absorbent cores in which a roll or coil of pre-formed absorbent core material is unwound directly onto the absorbent pad-making machine without the defiberization step required for fluff-based products. The roll wood-based approach results in a product thinness, which cannot be achieved by loose fluff-based technology. As will be seen later, the present invention can produce thinner absorbent products that have comparable or improved absorbency properties to thicker products. The present invention thus serves to further reduce product thickness and weight.
Although a given SAP particle has the capability to absorb and contain a liquid, in actual practice it is difficult to efficiently utilize this capability. If the SAP particles are located as a mass in close proximity to each other, the rate of absorption and the capacity to retain absorbed liquid are reduced because the liquid will not be able to reach, or only slowly reach, the interior of the SAP mass. As the outer surface area of the SAP mass begins to absorb liquid, in effect, a barrier is created that substantially slows the rate of liquid absorption. Consequently, it is understood that the separation, as well as the size, of the SAP particles are important considerations. Attempts to address this problem have included adhering the SAP particles to non-SAP material by hot melt adhesion or the use of resinous binders, including both thermoplastic and thermosetting types. There are problems associated with these attempts. The use of such adhering materials causes encapsulation or partial encapsulation of the SAP particles and thereby prevents or reduces the surface area available for efficient liquid absorption. In addition, free volume for particle expansion is compromised. Furthermore, even when the SAP particles are initially adhered, as the particles expand and soften during the absorption phase, the particles tend to separate and migrate toward other SAP particles thereby further reducing liquid absorption and free volume. The above prior art problems are beneficially addressed by the free volume considerations underlying the present invention.
In any event, many products for absorbing liquids and processes for making thereof are described in the art. Typical of such products and processes are those described in Assignee's co-pending U.S. patent application Ser. No. 10/357,907, filed Feb. 4, 2003, and published on Sep. 18, 2003, under Publication No. US-2003-0175418-A1; U.S. Pat. No. 5,792,513; U.S. Pat. No. 6,485,813; U.S. Pat. No. 5,611,885; and U.S. Pat. No. 6,534,572. However, none of these patents or patent publications is believed to possess or teach the unique combination of advantages of the present invention.
In general, the absorbent product of the present invention may utilize SAP particles having a predominant amount of particles on the order of a maximum average particle size of about 500 microns and a minimum average particle size of about 100 microns and thereby more rapidly absorb liquids, such as water, body fluids, urine, blood, etc., than possible when using coarser particles of the same weight. However, it should be understood that lower sized particles may be mixed with larger sized particles to further increase the rate of liquid absorption. Such increased rapidity of absorption is due to the larger total surface area of the fine particles. Rapid absorption is particularly important when the absorbent product is a portion, such as a core, of a diaper product or the like. The present invention also involves spacing absorbent SAP particles apart from each other and provides the necessary free volume surrounding the particles to permit the particles to expand more readily during absorption of a liquid to improve absorption capacity, rate, and efficiency and thereby permitting the use of less SAP particles than if such SAP particles were not extended through the sheet surface(s) and not so spaced apart and did thus not have as much free volume into which to expand. Creating and maintaining free volume also has the advantage of providing a space to collect and store liquids, such as water, for subsequent dispensing to, for example, plant root systems. Other processes involving adhering SAP particles to substrates have the distinct disadvantage of covering, or even encapsulating, a surface portion of the SAP particles, thereby reducing the effectiveness of the SAP particles.
While the present invention is primarily described in connection with diaper products, it will be understood by those skilled in the art that the product of the invention may be used in water collection, storage, and dispensing applications including irrigation, erosion control, and for plant watering and plant root growth control